It is well known that the external surfaces of glass used in automobiles and other motor vehicles accumulate two general forms of foreign matter during operation of the vehicle. One form, generally termed "visible dirt," consists of dust, mud, residues from insects, and the like. The other form consists of a hydrophobic film (often called road film) composed primarily of organic material, such as unburned fuel, lubricating oil, products of fuel combustion, and the like, emitted from other vehicles, and/or thrown up from the road surface by passage of other vehicles. This form also is often termed "invisible dirt" because it is not readily noticeable under many lighting conditions where the intensity of the light from the surroundings does not differ greatly from that of the interior of the vehicle. The dirt becomes highly visible, however, when the light intensity on the two sides of the glass differs greatly -- from a darkened vehicle, the film causes the glass to tend to act as a mirror; approaching a brilliant light source, such as the sun near the horizon, or the lights of an oncoming vehicle, the film increases "glare" and also may cause the glass to tend to act as a mirror. In both situations, visibility through the glass is markedly reduced, increasing the danger of operating the vehicle. Another situation when the "invisible dirt" becomes highly visible -- to the detriment of vision through the glass -- is when condensed moisture appears on the outer surface of the glass, the wiping action of the windshield wiper, or a cloth used to remove the moisture, causes a smearing action resulting in streaking or "fogging" of the glass.
It is relatively easy to remove the "visible dirt" but very difficult to remove the "invisible dirt."
These glass contamination problems are discussed in some detail in such patents as U.S. Pat. Nos. 2,313,425, 3,304,264 and 3,309,321.
To be useful for cleaning such glass surfaces, a cleaning composition thus must be able to remove the invisible dirt, as well as the visible dirt. Moreover, the cleaner must be capable of use on the vehicle under the expected range of climatic conditions -- it must not freeze at the low temperatures, nor evaporate at unduly high rates at the high temperatures that may be encountered in use of the vehicle -- and it must be compatible -- that is, it must not corrode metals it contacts, such as the parts of the washing equipment and metal trim around the glass, or adversely affect the body finish of the vehicle. Furthermore, from the marketing standpoint, it is very desirable that these characteristics appear in a single composition rather than requiring a series of compositions each of which is adopted only for a use under particular climatic conditions. Thus, it is common practice to market an "all-weather" composition for use in both summer and winter. Such a composition ordinarily is sold as a concentrate which is suitable for use in winter -- i.e., it has a low freezing point. The concentrate then is diluted with water to prepare a composition suitable for summer use -- it has a relatively high freezing point, but relatively low volatility, since if the composition dries too quickly, it may dry before cleaning of the glass has been effected or may interfere with the cleaning. Also, of course, it is always desirable to use as dilute a composition as possible, employing minimum amounts of chemicals to reduce contamination of the environment and to reduce the cost of using the composition.
Further, for convenience, ordinary tap water is usually employed as the diluent. Since this may be "hard" water containing dissolved calcium salts, it is desirable that the cleaning composition not be affected by such salts -- i.e., that no precipitated material form and no change in the cleaning properties result from addition of such hard water to the cleaning composition.